“Moisture sensitive foods” are those foods having transpiration rates that can cause high humidity conditions within a food package, causing moisture condensation, microbial growth, and decay of the product. Exemplary “moisture sensitive foods” include fresh produce and some moist bakery products (e.g., dough). Thus, packaging of moisture sensitive foods should permit the right mix of oxygen, carbon dioxide, and water vapor within the food package. In this regard, packaging material with a “high” moisture vapor transmission rate (MVTR) (that is, greater than 50 g/m2/day at 90% RH and at 23° C.) allows the packaging material to breathe so that moisture vapor inside the package does not turn into condensation. Condensation fogs the food package so a consumer cannot view the package contents and potentially leads to microbial growth and decay, thereby reducing the quality, safety, and shelf life of the food product. As used herein, “moisture vapor transmission rate” or “MVTR” generally is a measure of the egress of water vapor through the packaging material. “Shelf life” is the length of time before a food becomes unsuitable for sale, use, or consumption. The problems caused by high humidity conditions within food packaging are exacerbated at low temperatures. Therefore, packaging in high MVTR packaging material is especially important for packaging moisture sensitive foods stored, transported, and/or marketed at refrigeration temperatures.
Packaging in heat sealable packaging material is also important, for example, to ensure product confinement and protection against outside elements. Unfortunately, packaging material with a high MVTR may not be heat sealable. For example, nylon film has a MVTR value of about 260 g/m2/day for 25 μm thickness at 23° C. and 65% RH, rising to over 400 g/m2/day at 23° C. and 90% RH, but is not heat sealable. Further, most heat sealable polymers are polyethylene- or polyolefin-based, characterized by low moisture vapor transmission rate (MVTR), making them unusable for packaging moisture sensitive foods. For example, poly-vinylidene chloride (PVdC), a halogenated polyolefin, has been used as a heat sealable film or coating, but its high moisture barrier (i.e., a low MVTR) has made it unsuitable for packaging moisture sensitive foods. The MVTR of the heat sealable films or coatings do not change with a change in humidity.
In an attempt to overcome these problems, high MVTR substrates such as nylon film have conventionally been coated with a heat sealable coating such as acrylic, polyurethane, or the like or laminated with a heat sealable film. However, conventional heat sealable coatings and films reduce the MVTR of the substrate. In addition, the off-line coating and lamination processes are costly, as the “off-line” coating and lamination processes first form the substrate into a roll and then require the substrate to be unwound to coat or laminate it. Adhesion of the heat sealable film or coating to the substrate also is difficult to control, often requiring the use of a primer to ensure adhesion.
It is therefore desirable to provide a heat sealable food packaging film that has a high MVTR. It is also desirable to provide a method for producing such a heat sealable food packaging film. It is further desirable to provide a method for producing a heat sealable food packaging film that will be less costly than conventional production methods and eliminate off-line processing and the need for a primer. It is also desired to produce a heat sealable food packaging film that extends food product storage and shelf life. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.